1. Field of the Invention
This invention relates to a heat pump, and, more particularly, to a heat pump suitable for air conditioning vehicles utilizing beds of hydride forming materials for absorption and desorption of hydrogen as the working fluid of the system.
2. Description of the Prior Art
There has been an increased emphasis on the elimination of freon as the working fluid for air conditioning systems because of the contamination of the earth's atmosphere by escaping freon. This has led to the search for other refrigerants and to the investigation of improved air conditioning systems. The continuing energy crisis has also led to the search for the better use of fuels with minimization of waste and pollution. These factors have led, directly or indirectly, to efforts in developing a heat pump utilizing hydrogen as the working medium. Likewise, specific efforts have been directed to the development of a hydrogen heat pump for vehicle air conditioning, and this is the specific area to which the present invention is directed, although it will be apparent that the invention has application to hydrogen heat pumps in general, both for heating and cooling.
Unlike conventional air conditioners, which are powered by mechanical or electrical energy to drive the expansion of freon gas, the hydrogen heat pump is a chemical heat pump utilizing the heat of reversible reaction of hydrogen gas with selected intermetallic compounds. These hydrogen absorbing materials are usually referred to as metal hydrides, although the materials themselves are metal hydride forming materials. It has been known for a long time that the safest way of storing large quantities of hydrogen gas is with the use of these metal hydrides.
It is also well known that these materials include certain metals and intermetallic compounds which form metal hydrides when they are exposed to hydrogen gas under appropriate temperature and pressure conditions. The chemical equation describing this reaction is: EQU M+(x/2)H.sub.2 .revreaction.MH.sub.x
where M is the hydride forming material. The absorbing material breaks up into fine powder due to hydrogen gas penetrating and expanding the crystalline lattice of the material. The direction of the shift from equilibrium between the material and hydrogen is determined by the thermodynamics of the reaction, i.e., the temperature and gas pressure determine whether absorption or desorption takes place. By selecting a pair of different hydride forming materials, one more stable to hydride formation than the other, a system can be developed to absorb and desorb hydrogen from one hydride forming bed to another by thermally cycling one of the two beds. The hydride beds can be incorporated into heat exchange or reaction vessels to constitute the evaporator and condenser components of the heat pump air conditioning system.
The development of a hydrogen heat pump for use in air conditioning vehicles has been implemented by the development of such a system for air conditioning a bus, as set forth in U.S. Pat. No. 4,436,539 to Moshe Ron et al. One of the primary problems is the appropriate selection or development of a higher temperature, more stable hydride forming material and a compatible lower temperature, less stable hydride forming material. For example, it is readily apparent that the two materials must be chosen so that the desorption pressure of the lower temperature, less stable metal hydride forming material is higher than the absorption pressure of the higher temperature, more stable metal hydride forming material, and that the desorption pressure of the higher temperature, more stable metal hydride forming material is higher than the absorption pressure of the lower temperature, less stable metal hydride forming material at the temperatures to which the materials are exposed in accomplishing the desired degree of cooling with the available waste heat regenerating temperature. This patent application is directed in part to the development of new metal hydride forming materials for solving this problem in an economically feasible fashion.
A second equally important problem is the construction of a bed of a hydride forming material which will have a high enough heat transfer rate and will have sufficient physical stability to be economically feasible for use in a vehicular environment. Since the beds of hydride material must be thermally cycled in use, and the hydride is normally in the form of fine powder having a very low thermal conductivity, the heat exchanger design is extremely critical and must provide intimate contact with the material and the means to conduct heat to and from the bed to the external heat transfer mediums being used. The problem is further compounded by the fact that the absorbing or hydrogenation process itself tends to break down the already fine material into even finer particles, due to the expansion of the hydrogen in the crystalline lattice of the material. The fine material tends to cause plugging of filters being used to confine the material with a corresponding increase in pressure drop and reduction in hydrogen flow. The solution to this problem offered in the aforementioned Ron et al. U.S. Pat. No., 4,436,539, is to compact the hydride material with a finely divided matrix metal powder which is deformable into a porous body structure, with the intimate contact of the powder metal offering a sufficiently high heat transfer rate to a confining vessel. The process of preparing such a compact of finely divided metal having a high heat transfer rate and the metal hydride powder is set forth in U.S. Pat. No. 4,292,265 to Moshe Ron et al. Unfortunately, these hydride compact pellets are not commercially viable since they break down by spalling in something less than fifteen absorption/desorption cycles, as stated by the examples cited in U.S. Pat. No. 4,292,265. The present invention is directed to the solution of this problem of providing a high enough heat transfer rate and a long enough physical stability to make the bed economically feasible.